High pressure (HP) turbine rotor blades and stator vanes of gas turbine engines can be subject to undesirable non-uniform velocity and temperature distributions in the working gas exiting from the combustor. In particular, circumferentially spaced “hot streaks” can be formed in the working gas, each streak extending downstream and originating from one of the circumferentially arranged fuel injectors of the combustor
Circumferential non-uniform temperature distribution can affect the heat load on the blades in both the first row of nozzle guide vanes (NGVs) and the following rotor blade row.
A design parameter, termed “clocking”, that can influence the NGV heat load is the relative circumferential positioning between the peak temperature of a combustor exit temperature profile (i.e. the centre of a hot streak) and a given NGV. The clocking effect depends on the respective fuel injector and NGV counts. A typical injector-NGV count ratio is 1:2, i.e. one injector corresponds to a pair of vanes.
FIGS. 1(a) and 2(a) show schematically respective working gas temperature distributions at the combustor exit of an engine, the temperature distributions being superimposed on a view from the front of a pair of NGVs. FIGS. 1(b) and 2(b) show schematically respective midspan sectional views of the NGVs and the corresponding temperature distributions at midspan. The centre of a hot streak corresponds to the peak temperature in each distribution. As shown in FIGS. 1(a) and (b), the hot streak can be aligned to impinge on the leading edge of one of the NGVs. In this case, the impinged vane will be subject to a higher heat load than the other NGV of the pair, but an advantage of such an arrangement is that the heat load built produced by the hot streaks on the blade pressure surfaces in the following rotor can be countered to an extent by a “negative jet” associated with NGV wake, the jet causing a pressure surface (PS) side to suction surface (SS) side movement in the working gas. Alternatively, the hot streak can be aligned in the middle of a NGV passage, as shown in FIGS. 2(a) and (b) to produce a more equal heat load on the NGVs.
One option for managing non-uniform heat load on the NGVs is to have different cooling arrangement for the NGVs, so that NGVs exposed to higher heat loads are subject to additional cooling. However, the non-equal cooling for the two NGVs introduces a non-equal aerodynamic flow field, which may prove to be detrimental to aero-thermal performance.
Another option is to shorten the chord length of some NGVs and position these NGVs so that their leading edges are further downstream than the other NGVs. The shortened NGVs can thus effectively be thermally shielded by the adjacent longer NGVs. However, the long and short NGV arrangement can also produce flow non-uniformity which may be detrimental to aerodynamic performance.
These detrimental effects can be exacerbated when there is a strong aerodynamic non-uniformity at the NGV inlet, such as a swirling flow and a non-uniform turbulence intensity, with peak turbulence typically at hot streak centres.